Power distribution systems are designed to handle peak consumer load, while still sparing some capacity to cover for contingency overloads and for projected growth of load. Under steady state operation, at any time, the load on the system depends upon the number of consumer appliances/loads that are in operation. A single end user imposes very high variability of load on the power system. Since different appliances are turned on/off at different times, the demand for electricity is highly erratic. However, the electric distribution grid or the distribution feeder sees the aggregate demand for electricity, which is smoothed out by demographic and temporal diversity. For distribution systems that provide power to a relatively large and diverse group of consumers, the peak electricity demand on the feeder may be 20-50% of the combined undiversified peak demands. An example would be the operation of the HVAC or air conditioning units that operate based on temperature set points, temperature dead bands, and duty cycles. With many thermostatic loads online, there is enough randomization in the above three parameters that will cause these units to turn on/off at different times, thus reducing the maximum instantaneous power demand compared to a scenario where a single HVAC system of equivalent size was operated. Furthermore, there is also appliance diversity within individual customer demand. This diversity is lost after an extended outage, which creates a sudden, large, undiversified power demand on the feeder that is much larger than the typical diversified feeder load.
When the system is not in a steady state operating condition, problems can arise due to loss of load diversity. One such condition is when the system is recovering from an extended period of outage or a cold load pickup event. Under such a situation, the feeder is required to respond to a very high surge in electricity demand because the temporal factor or time diversity in power demand (from the end users) is lost. This is because after an extended outage all of the appliances come online at the same time and demand their maximum rated power until their steady state is reached. For example, in the case of thermostatic loads, as the desired steady state temperature can be very different from the present temperature, especially after an extended outage, such appliances will remain in operation until the desired temperature is achieved. As a result, the power distribution system may see a large spike in power demand for an extended period of time. In addition, the energization of a distribution circuit may result in high in-rush currents due to transformer magnetization and motor starting. This further compounds to the peak current demand. Under some conditions, the over current relay may react to such high overloads, triggering circuit breakers to open the distribution feeder. Such triggering and redo sing of the distribution feeders is a nuisance and can cause delays in load pickup. Further, large currents flowing on the feeder for an extended period of time can adversely affect equipment life.
A lot of research has gone into development of strategies to address cold load pickup. Most of the strategies control the power demand under a cold load pickup event by energizing pockets of load, one at a time. The loads are grouped/sectionalized into distinct pockets and the power is restored to one pocket at a time. The energization of a load pocket is done after a certain time delay from the energization of the previous one. With such an approach, the power to the last group of loads can only be restored after a delay, when all the previous groups have been energized.
For these and other reasons, there is a need for the present invention.